1. Field of the Invention
The present invention relates to a display device and a manufacturing method thereof, and more particularly to a display device in which thin film transistors are formed on a substrate and a manufacturing method thereof.
2. Description of the Related Art
An active-matrix-type display device is configured as follows. Respective pixels are arranged in a matrix array. By supplying a scanning signal to a signal line (gate signal line) shared in common by the respective pixels arranged in a row direction, a group of pixels arranged on the same row is selected. Then, the signal lines which supply the scanning signal are sequentially scanned in a columnar direction (from top to bottom in a display region, for example). In conformity with such selection and scanning timing, a video signal is supplied to the respective pixels arranged in the columnar direction through a signal line (drain signal line) shared in common by the respective pixels.
For this end, each pixel includes a thin film transistor which is a switching element for supplying a video signal from the drain signal line to the pixels (pixel electrodes) in response to the supply of the scanning signal.
Further, on the same substrate on which the pixels are formed, a drive circuit which supplies a scanning signal to the gate signal lines and supplies a video signal to the drain signal lines is mounted. The drive circuit is also constituted of a circuit which includes a plurality of thin film transistors.
Here, the thin film transistor is constituted of a so-called MIS (Metal Insulator Semiconductor) transistor in which a gate insulation film is formed so as to cover a gate electrode, a semiconductor layer is formed on an upper surface of the gate insulation film in a state where the semiconductor layer straddles the gate electrode, and a pair of electrodes (a drain electrode and a source electrode) which are arranged in an opposed manner with a region above the gate electrode (channel region) sandwiched therebetween is formed on an upper surface of the semiconductor layer.
Further, there has been known a thin film transistor where portions which connect a semiconductor layer and respective electrodes include a high-concentration impurity layer (n+ layer) as a contact layer, and a low-concentration impurity layer (n− layer) having the same conductive type as the high-concentration impurity layer is formed on a channel region side of the high-concentration impurity layer. The low-concentration impurity layer is referred to as a so-called LDD (Lightly Doped Drain) layer and, for example, is provided for alleviating the concentration of an electric field between the contact layer and the gate electrode.
A display device having such a thin film transistor is disclosed in JP-A-10-96956 (patent document 1), for example.